This invention relates to the creation of Bragg reflection gratings in photosensitive optical fibres. It is known that such a grating can be created by illuminating the photosensitive optical fiber from the side, writing the lines simultaneously with an interferometrically generated fringe pattern of light. Such a fringe pattern can be generated using two-beam interferometry, or as a localized fringe pattern generated in the vicinity of a diffraction grating, typically a phase grating, through which the light is caused to pass.
Typically relatively high intensity UV light is employed to write such Bragg gratings and, although relatively short gratings can be written in fiber using two-beam interferometry with the requisite uniformity, for applications requiring longer gratings extending over at least a few centimeters, the use of a phase grating mask to generate the requisite fringe pattern is generally preferred because the light does not have simultaneously to illuminate the whole length of fiber to be written, but can be traversed in a small, and hence higher intensity, beam.
A Bragg grating of uniform pitch can be generated using a phase grating also of uniform pitch. Similarly a chirped Bragg grating a grating, the optical pitch of whose grating elements varies along its length, can be generated using a phase grating of non-uniform pitch. (The optical pitch is the product of the physical pitch with the effective refractive index of the fiber. The effective refractive index is generally, but not necessarily, uniform along the length of the fiber: it could be non-uniform, for instance, if the fiber were tapered.)
The construction of a chirped Bragg grating in a tapered fiber using a uniform pitch two-beam interference generated fringe pattern is described in the specification of GB Patent Application No 9316738.5
A chirped Bragg grating can also be generated using a phase grating of uniform pitch by the method described by J Martin et al in a paper entitled `Novel writing technique in long in-fiber Bragg gratings and investigation of the linearly chirped component` CLIO conference paper Monday, May 11, 1994, this method involving maintaining a temperature gradient along the fiber while the grating is written into it.